ILS in Aviation

Pilots · 4 min read · Jan 03, 2022
ils aviation

One of the most complex stages of any flight is approaching the runway and landing at the end of it. Doing this properly is critical for the safety of the pilots, their crew, and passengers. Sometimes, this must be done with visual limitations that prevent the pilot from using any external visual references.

Fortunately, ILS systems exist to help pilots achieve a safe approach and descend towards the landing area in any situation. ILS stands for Instrument Landing System, so let’s see what ILS systems are.

Instrument Landing System (ILS)

An Instrument Landing System is defined as a group of navigation or precision approach assistance to safely guide the aircraft towards the runway. The Instrument Landing System (ILS) is a precision aviation instrument based on radio beams that provide the pilot with both lateral and vertical guidance during the approach and landing stages of the flight.

An instrument landing system has various essential components for proper operation, and each of them has a specific role. These components are usually categorized in navigation, range, and visual, and the most important ones are the localizer and the glide-slope. Keep reading as we show you the details of the ILS components.

View of the primary component of the ILS, the localizer at the runway 06L of the Montréal–Trudeau International Airport, Canada.
Image source:

The main components

As it was mentioned above, there are two that are the most important components for the ILS approach and landing. However, we want you to learn about all of them, so we describe them here below.

ILS Localizer

This is the primary component of the ILS approach. The localizer provides the lateral guidance for the pilot to align the aircraft with the runway properly. This is done by transmitting two narrow beams, one that goes moderately to the right and the other to the left of the centerline of the runway, and that intersect in that centerline. The intersection is known as the “on LOC” signal or indicator.

The pilot receives information regarding how far the aircraft is from the runway centerline via airborne equipment usually located at the opposite end of the runway threshold.

ILS Glide-slope for vertical guidance

While the localizer offers lateral guidance, the ILS glide slope is vertical. This helps the ILS approach system to provide information regarding the altitude for the pilot to land the aircraft safely.

Similarly, the GS airborne equipment uses two narrow beams that intersect, creating the “on GS” signal. The beams move moderately above and below the standard vertical profile that has been established for a safe approach and landing. To do so, the GS airborne equipment provides a crossing height of about 50 ft at the runway threshold, with a standard angle of 3 degrees. However, the angle may vary from an airfield to another as there may be different constraints, including obstacles or ground formations.

Marker Beacons

Certain ILS installations will include marker beacons. These beacons are in the category of range assistance since they allow the pilots to know better how far they are from the runway. A marker beacon works by flashing colored lights and morse code tones, and groups of them are normally in two positions when installed, the outer marker and the middle marker.

The outer marker beacon is the typical name of the first beacon, which is usually located 3 1/2 to 6 NM from the threshold within 250 ft of the extended runway centreline to allow the pilot to make a positive position fix on the localizer.

On the other side, the middle marker beacon is located approximately 0.5 to 0.8 NM from the threshold on the extended centerline of the runway. The middle marker crosses the GS at approximately 200 to 250 ft above the runway elevation.

Both beacons allow the pilot to adjust the aircraft’s position before the final approach to the runway.

Distance Measuring Equipment (DME)

The DME is one of the alternatives of navigation assistance that can be added to the system together with the localizer course. The DME helps the pilot learn the distance from the runway as it provides slant distance to the aircraft concerning touch-down point.

Approach Lighting System

This component is also known by the acronym ALS, and it belongs to the visual category. The main purpose is to make the runway as noticeable as possible, even in the lowest visibility situations. Other lighting systems used in ILS for visual reference include:

  • Sequenced Flashing Light (SFL)
  • Touchdown Zone Lights (TDZ)
  • Centerline Lights (CLL)

Runway Visual Range (RVR)

These are essential components that act as a limit for the pilot to complete the final approach and land. They take the form of altitude levels where the minimum visual reference must be available. Therefore, the RVR will depend on the type of precision instrument approach procedure used or the category of the ILS signals.

In general, there are two altitudes defined based on the types of approach.

The first is called the Minimum Descent Altitude (MDA), which is used for a type of approach known as the non-precision approach, or an approach that does not have the full ILS with LOC and GS included. The MDA defines the minimum altitude for the pilot to level off and stabilize the aircraft before starting the approach to land.

The second one is the Decision Height (DH) used in precision approaches using GS. The DH defines the minimum altitude for the pilot to determine whether the runway environment is visible enough to continue with the final approach course and engage normal landing. Whenever the pilot identifies the visibility below the minimum established in the approach chart, a missed approach is initiated, and the name of Missed Approach Point (MAP) is used for the DH in these cases.

Based on the standard category of ILS or Category I, the standard DH is 200 feet above runway threshold elevation (ARTE), and the RVR is 550 meters. This means the pilot can take the aircraft to 200 feet without any outside visual reference beyond the 550 meters of distance. However, the higher the category, the lower any aircraft can go since the runway provides more accurate guidance. For example, category II permits a DH of not less than 100 ft and an RVR of 300 meters.

A view of a runway, an instrument landing system (ILS) and an aircraft ready to land at an airport.

Final Thoughts

ILS are essential systems to help pilots perform precision approaches safely. These navigation aids also ensure air traffic flow smoothly since fewer missed approaches are needed, and the landings are achieved.

However, these systems required very specialized equipment and suitably equipped aircraft to work properly, thus making it expensive for smaller airports to reach ILS category II or higher and sometimes too expensive even to have ILS installed. Nevertheless, these are commonplace in the biggest airports of the world.

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Jet pilot @NASA

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